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Transient photocapacitance spectra for five CZTSSe samples obtained at 220 K and 5 kHz. (a) Comparison of the 3 lower band-gap samples. The solid lines represent fits to the optical spectra based upon transitions into a narrow band-tail distribution of states plus a deeper, roughly gaussian shaped defect band centered near 0.8 eV. The dashed lines indicate the underlying density of states for sample 3. The small arrows at the top indicate estimates of the optical gaps for these samples based on these TPC spectra. (b) Comparison of the TPC spectrum for the 2 higher band-gap samples along with a replot of the sample 2 TPC spectrum for comparison. For these higher band-gap samples, the band-tail is much broader. The energy distribution of the deeper band of transitions is less clear. Note that the scales in (a) and (b) are slightly different.
The open-circuit voltage, VOC, vs. EU (the triangles and left hand scale) together with the voltage deficit vs. EU (using the right-hand scale). The “voltage deficit” is the difference: Eg /q − VOC . It was obtained two ways: (1) By estimating Eg from EQE data (squares) and, (2) By estimating Eg from the arrow positions in Fig. 1 (circles), which denote the end of the exponential band-tail regime. While the two estimates for Eg differ (for reasons discussed), the slopes are nearly identical.
Cell performance parameters for the 5 sample devices studied together with the values of EU determined by the TPC spectra. The values of Eg listed are estimated from the long wavelength inflection points of the cell EQE curves. The density of deep acceptors obtained by DLCP, as given elsewhere,6 are also included for reference.
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